Notes for php learning NOTE _ 4 _ constants and magic constants

: This article mainly introduces the things you need to pay attention to when learning the _ 4_constant and the magic constant in php. if you are interested in the PHP Tutorial, please refer to it. In php, constants correspond to Variables. constants are subsystem constants, framework constants, and custom constants.
You can use the following to define constants:

Note:
1. use it directly without the '$' symbol;
2. constants can be defined and used anywhere;
3. you can use the constant name or the constant (constant name) function to obtain the constant value;
4. get_defined_constants (); you can get all defined constants;

Magic constants: they are added by different extension libraries. Common magic constants include:

Name	Description
__LINE__	The current row number in the file.
__DIR__	Directory of the current file.
__FILE__	The complete file path and file name. If it is used in a include file, the include file name is returned. Since php 4.0.2, FILE always contains an absolute path, while previous versions sometimes contain a relative path.
__FUNCTION__	Function name (new php 4.3.0 ). Starting from php 5, this constant returns the name (case sensitive) when the function is defined ). In php 4, the value is always lowercase letters.
__CLASS__	Class name (new php 4.3.0 ). Starting from php 5, this constant returns the name (case sensitive) when the class is defined ). In php 4, the value is always lowercase letters.
__METHOD__	Class method name (new php 5.0.0 ). Returns the name (case sensitive) when the method is defined ).
__NAMESPACE__	Name of the current namespace (case sensitive ). This constant is defined during compilation (new in PHP 5.3.0)

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The above describes the things that need to be paid attention to when learning the _ 4_constant and the magic constant in php, including some content. I hope to help anyone who is interested in the PHP Tutorial.